Between the election campaign and incidents of terrorism ranging from attempted bombings to a synagogue shooting, an obscure regulatory proposal by the Trump administration has yet to captivate the public’s attention. However, it has the potential to change the way millions of Americans obtain health insurance.

In the United States, unique among industrialized countries, most Americans under age 65 receive health coverage from their employers. This occurs largely due to an Internal Revenue Service (IRS) ruling issued during World War II, which excluded health insurance coverage from income and payroll taxes. (Businesses viewed providing health insurance as one way around wartime wage and price controls.)

But providing coverage through employers creates problems: It keeps some people locked to their job just for the health insurance, and it harms people who have to stop working, and therefore lose their coverage, because of ill health. That’s one of the big reasons pre-existing conditions have become a political football.

The Trump administration’s proposed rule would, if finalized, allow businesses to make a pretax contribution towards individual health insurance—that is, coverage that individuals own and select, rather than employers. This change may take time to have an impact, but it could lead to a much more portable system of health insurance—which would help to solve the pre-existing condition problem.

How Would It Work?

Under the proposed rule, employers could provide funds through a Health Reimbursement Arrangement (HRA) to subsidize the purchase of individual health insurance. Employers could provide the funds on a pretax basis, and—provided that the workers purchase their coverage outside of the Obamacare exchanges—employees could pay their share of the premiums on a tax-free basis as well.

In practical terms, some employers may choose to provide a subsidy for health coverage—say, $300 per month, or $5,000 per year—in lieu of offering a firm-sponsored health plan. Individuals could go out and buy the plan they want, which covers the doctors whom they use, rather than remaining stuck with the plan their employer offers. And employers would get better predictability for their health expenses by knowing their exposure would remain fixed to the sums they contribute every year.

Small businesses with fewer than 50 full-time workers can already use this defined-contribution form of health coverage. A provision in Section 18001 of the 21st Century Cures Act, signed into law by President Obama in December 2016, permitted small firms to use HRAs to reimburse their workers’ individual health insurance premiums. The proposed rule would extend this freedom to all employers.

Could Employers Game the System?

The proposed rule acknowledged the possibility that employers might try to “offload” their costliest patients into individual health coverage, lowering expenses (and therefore premiums) for the people who remain. The rule contains several provisions designed to protect against this possibility.

Employers must choose to offer either an HRA contribution towards individual coverage or a group health plan. They cannot offer both options, and whatever option they select, they must make the same decision for an entire class of workers.

A “class” of workers would mean all full-time employees, or all part-time employees, or all employees under one collective bargaining agreement. Hourly and salaried workers would not count as separate “classes,” because firms could easily convert workers from one form of compensation to another. These provisions seek to ensure that firms will offer some employees health insurance, while “dumping” other employees on to individual coverage.

In addition, employers may vary contributions to the HRA only by defined characteristics—for instance, providing a larger contribution to older workers, or workers with larger families. Again, the rules intend to minimize the extent to which businesses can try to discriminate or encourage workers to choose one form of coverage over another.

Can Workers Buy Short-Term Coverage with Employer Funds?

However, the rule creates a separate type of account to which employers could contribute that would fund workers’ “excepted benefits.” This term could include things like long-term care insurance, vision and dental insurance, and the new short-term plans the Trump administration has permitted. But employers could only fund these accounts up to a maximum of $1,800 per year, and they could create these special “excepted benefits” accounts only if they do not offer an HRA that reimburses workers for individual insurance, as outlined above.

Will Firms Drop Health Coverage?

Some firms may explore the HRA option over time. However, the extent to which businesses embrace defined-contribution coverage may depend upon the viability of the individual health insurance market, and the status of the labor market.

Right now, many counties have only one health insurer offering Obamacare-compliant plans, and unemployment remains at or near record lows. With many businesses desperate to attract and retain qualified workers, it seems unlikely that many firms offering coverage now will drop their plan to offload people on an individual market with limited options.

However, if and when more insurers return to the marketplace, firms may view the defined-contribution method of health coverage as a win-win: employees get more choices and employers get predictability over health costs. Particularly if unemployment ticks upward, or one firm in an industry makes the move towards the HRA model, other businesses may follow suit in short order.

Will the Proposal Cost Money?

It could. The proposed rule should cost the federal government $29.7 billion over the first ten years. That estimate assumes that 800,000 firms, offering coverage to 10.7 million people, will use the HRA option by 2028. (It also assumes an 800,000 reduction in the number of uninsured Americans by that same year.)

The cost, or savings, to the federal government could vary widely, depending on factors like:

Whether firms using the HRA option previously offered coverage. If firms that did not offer coverage take the HRA option, pretax health insurance payments would increase, reducing tax revenues. (The rule assumes a reduction in income and payroll tax revenue of $13 billion in 2028.)

Whether individuals enrolling in individual market coverage via the HRA option are more or less healthy than current enrollees. If the new enrollees are less healthy than current enrollees, individual market premiums will rise, as will spending on Obamacare subsidies for those individuals. (The rule assumes a 1 percent increase in individual market premiums, and thus exchange subsidies.)

The extent to which HRAs affect eligibility for Obamacare subsidies. If some low-income individuals whose employers previously did not offer coverage now qualify for HRA subsidies, they may lose eligibility for Obamacare subsidies on the exchanges. (The rule assumes a reduction in Obamacare subsidies of $6.9 billion in 2028.)

Given the many variables in play, the rule has a highly uncertain fiscal impact. It could cost the federal government billions (or more) per year, save the federal government similar sums, or have largely offsetting effects.

Ironically, the Trump administration represents but the most recent Republican presidency to examine the possibility of defined-contribution health insurance. While working on Capitol Hill in 2008, I tried to encourage the Bush administration to adopt guidance similar to that in the proposed rule. However, policy disagreements—including objections raised by, of all places, scholars at the Heritage Foundation—precluded the Bush administration from finalizing the changes.

Since I’ve fought for this concept for more than a decade, and included it in a series of regulatory changes the administration needed to make in a paper released shortly before Trump took office, I can attest that this change is as welcome—and needed—since it is overdue. Although overshadowed at the time of its release, this rule could have a substantial effect on Americans’ health insurance choices over time.

Mr. Jacobs is founder and CEO of Juniper Research Group, a policy consulting firm based in Washington. He is on Twitter: @chrisjacobsHC.